Post what are the +12V rating of that PSU as that's what really matters for your rig; 380W isn't saying much, since that adds up all power available on +12V +5V and +3.3V rails, while the 9800GT & the P4 use only +12V. It might be that you oly have (say) 15Amps on the +12V which is 15x12=180W, 180W usable watts. 2 cards and a P4 at full speed consume more than that, so the PSU might be strained.

My take: post the +12V amp rating to see what your PSU can really do; also post a Speedfan screenshot while the rig is folding - this is a temp and voltage monitoring soft that should point out if the PSU is overloaded.